1. Field of the Invention
This invention relates to a hot rolled steel wire rod or bar for machine structural use and a method of producing the same. More specifically, it relates to the use of a soft steel wire rod or bar easily workable by drawing, machining, cold forging and other cold working methods, in an as-rolled condition without softening, in manufacturing components for cars and construction machines and the like, and a method to produce the same.
2. Description of the Related Art
Components of cars and construction machines and the like have been conventionally manufactured by softening a hot rolled steel wire rod or bar to secure its cold workability, forming it by cold forging, drawing, machining and/or other cold working methods, then quenching and tempering the pieces thus formed.
When manufacturing bolts, as an example of machine components, from a hot rolled steel rod, the cold workability is secured by applying softening in the following manner: a low temperature annealing at about 650xc2x0 C. for 5 hours in the case of stud bolts and the like, which require light cold working; a common annealing at about 700xc2x0 C. for 7 hours in the case of hexagonal bolts and the like; and a spheroidizing annealing at about 720xc2x0 C. for 20 hours in the case of flanged bolts and the like, which require heavy cold working.
As stated above, the softening process takes a long time and the cost of the annealing has come to account for a considerable portion in the total manufacturing costs of machine components and the like because of the recent rise in energy costs.
In this situation, various technologies to eliminate the softening process before the cold working have been proposed for enhancing productivity and saving energy such as the following: a production method of a low alloy steel excellent in cold workability disclosed in Japanese Unexamined Patent Publication No. S57-73123; a direct softening method of a steel wire or bar for a structure disclosed in Japanese Unexamined Patent Publication No. S58-58235; and a production method of a steel for machine structural use suitable for cold working disclosed in Japanese Unexamined Patent Publication No. H8-209236.
The cold workability of the steel wire rods or bars obtained by these production methods, as-hot-rolled, however, is insufficient compared with conventionally softened steel wire rods or bars and, thus, no soft steel wire rods or bars for machine structural use satisfactory for actual use in the as-hot-rolled condition have been obtained yet.
In view of the above situation, the object of the present invention is to provide an as-hot-rolled steel wire rod or bar for machine structural use having as good cold workability as the conventional steel wire rods or bars softened after hot rolling, and a method to produce the same.
The present inventors directed their attention to the structure of steel wire rods or bars obtained through a softening process and studied a method to secure cold workability by obtaining, in the as-hot-rolled condition, a structure equivalent to the one resulting from the softening.
FIG. 1 is a micrograph (xc3x974,000) of a hot rolled CH45K steel wire rod after a common softening treatment (700xc2x0 C.xc3x973 h.). As seen in the figure, the microstructure of the steel consists of ferrite and lamellar pearlite, and tabular cementite in the lamellar pearlite is partially cut into fragments. The softening of the steel is caused by a prescribed percentage of ferrite in its structure and the fragmentation of the cementite in the lamellar pearlite, and the cold workability of the steel wire rod is thus secured.
The present inventors discovered that steel wire rods or bars produced by hot rolling a billet, having a specific chemical composition at a low temperature and cooling in the temperature range from 700 to 650xc2x0 C. at an ultra-low cooling rate, had a novel structure wherein ferrite accounted for a high percentage and the cementite in the lamellar pearlite was partially spheroidized, and that good cold workability was thus secured since the steel wire rods or bars were soft in the as-hot-rolled condition thanks to the novel structure; and established the present invention on the basis of the finding.
The gist of the present invention, therefore, is as follows:
(1) A hot rolled steel wire rod or bar for machine structural use, characterized in that: the wire rod or bar is made from a steel consisting of, in weight,
0.1 to 0.5% of C,
0.01 to 0.5% of Si,
0.3 to 1.5% of Mn,
and the balance consisting of Fe and unavoidable impurities; its microstructure consists of ferrite and pearlite; its ferrite crystal grain size number defined under Japanese Industrial Standard (JIS) G 0552 is 11 or higher; and the granular carbide 2 xcexcm or less in circle-equivalent diameter and having an aspect ratio of 3 or less accounts for a percentage area of 3 to 15%.
(2) A hot rolled steel wire rod or bar for machine structural use according to the item (1), characterized by further containing, in weight, one or more of;
0.2 to 2.0% of Cr,
0.1 to 1.0% of Mo,
0.3 to 1.5% of Ni,
1.0% or less of Cu, and
0.005% or less of B.
(3) A hot rolled steel wire rod or bar for machine structural use according to the item (1) or (2), characterized by further containing, in weight, one or more of;
0.005 to 0.04% of Ti,
0.005 to 0.1% of Nb, and
0.03 to 0.3% of V.
(4) A method to produce a hot rolled steel wire rod or bar for machine structural use, characterized by subjecting a steel having the chemical composition specified in any one of the items (1) to (3) to a rough hot rolling in a temperature range from 850 to below 1,000xc2x0 C., a finish hot rolling in a temperature range from the Ar3 transformation temperature to 150xc2x0 C. above it and a cooling from 700 to 650xc2x0 C. at a cooling rate of 0.02 to 0.3xc2x0 C./sec., so that the steel may have a ferrite crystal grain size number defined under JIS G 0552 being 11 or higher and contain the granular carbide 2 xcexcm or less in circle-equivalent diameter and having an aspect ratio of 3 or less account for a percentage area of 3 to 15%.